Summary
The data generated from special-core-analysis (SCAL) tests have a
significant impact on the development of reservoir engineering models. This
paper describes some of the criteria and tests required for the selection of
representative samples for use in SCAL tests. The proposed technique ensures
that high-quality core plugs are chosen to represent appropriate flow
compartments or facies within the reservoir. Visual inspection and, sometimes,
computerized tomography (CT) images are the main tools used for assessing and
selecting the core plugs for SCAL studies. Although it is possible to measure
the brine permeability (kb ), there is no direct method for
determining the porosity (f) of SCAL plugs without compromising their
wettability. Other selection methods involve using the
conventional-core-analysis data (k and f) on “sister plugs” as a
general indicator of the properties of the SCAL samples.
A selective technique ideally suited for preserved or “native-state” samples
has been developed to identify reservoir intervals with similar
porosity/permeability relationships. It uses a combination of wireline log,
gamma scan, quantitative CT, and preserved-state brine-permeability data. The
technique uses these data to calculate appropriate depth-shifted
reservoir-quality index (RQI) and flow-zone indicator (FZI) data, which are
then used to select representative plug samples from each reservoir
compartment. As an example application, approximately 400 SCAL plugs from an
Upper Jurassic carbonate reservoir in the Middle East were tested using the
selection criteria. This paper describes the step-by-step procedure to select
representative plugs and criteria for combining the plugs for meaningful SCAL
tests.
Introduction
The main goal of coring is to retrieve core samples from a well to get the
maximum amount of information about the reservoir. Core samples collected
provide important petrophysical, petrographic, paleontological,
sedimentological, and diagenetic information. From a petrophysical point of
view, the whole-core and plug samples typically undergo the following tests: CT
scan, gamma scan, conventional tests, SCAL tests, rock mechanics, and other
special tests. The data are combined to get information on heterogeneity, depth
shift between core and log data, whole-core and plug porosity and permeability,
porosity/permeability relationship, fluid content (Dean-Stark), RQI, FZI,
wettability, relative permeability, capillary pressure, stress/strain
relationship, and compressibility. The petrophysical data generated in this way
play important roles in reservoir characterization and modeling, log
calibration, reservoir simulation, and overall field production and development
planning.
Among all the petrophysical tests, the SCAL tests (which include
wettability, capillary pressure, and relative permeability determination) are
critical and time-consuming. A reservoir-condition relative permeability test
can sometimes run for several months when mimicking the actual flow mechanisms
taking place in the field. Therefore, it is very important to design these
tests properly and, in particular, to select the samples that ensure meaningful
results. In short, the samples must be “representative samples,” which can
capture the overall variability within the reservoir in a more scientific way.
Unfortunately, the most important aspect of all SCAL procedures, the sample
selection, is one of those least discussed. According to Corbett et al. (2001),
API’s RP40 (Recommended Practices for Core Analysis) makes very little
reference to sampling; similarly, textbooks on petrophysics do not have
sections on sampling. The Corbett et al. paper reviewed the statistical,
petrophysical, and geological issues for sampling and proposed a series of
considerations. This has led to the development of a method (Mohammed and
Corbett 2002) using hydraulic units in a relatively simple clastic
reservoir.
In this paper, some issues related to sample-selection criteria (with
special focus on carbonate reservoirs) will be discussed. A large data set of
conventional, whole-core, and special-core analyses on a well in an Upper
Jurassic carbonate reservoir was used to characterize representative samples
for SCAL tests.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
9 February 2004
- Revised manuscript received:
27 May 2006
- Manuscript approved:
10 August 2006
- Version of record:
20 December 2006
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